It is well known to employ stress control means to control electrical stress due to a discontinuity in a region of high electric field strength high voltage electrical equipment, for example, electrical bushings, and joints or terminations of high voltage cables. Such stress control means typically comprise, stress cones and tapes or tubular articles of semiconductive stress control material. This invention is directed to stress control means comprising semiconductive stress control material and provides improved discharge extinction and impulse performance over prior art arrangements using such stress control means. For purposes of illustration, this invention is described primarily as it applies to a termination of a high voltage cable. The invention can be applied, however, to other electrical equipment where stress control is desired.
A typical high voltage cable includes an inner conductor surrounded by a conductor shield which is, in turn, surrounded by an insulating material that is surrounded by an outer electrically conductive shield and metal shield. The cable, typically also includes an outer protective cable jacket. In terminating such a cable, it is customary to remove or cut back each successive layer of the cable to expose the layer below. Cutting back the electrically conductive shield causes a discontinuity in the electric field resulting in high electric stress at the end of the shield. The high electrical stress can cause electrical discharges to occur, which in turn tend to cause breakdown of the insulation of the cable. The high electrical stress can be controlled by electrical stress control means.
High-voltage alternating-current cable terminations are generally tested in the U.S. under the IEEE standard test procedure Std. 48-1975. This procedure sets forth, inter alia, design tests to be performed by the manufacturer to obtain information on the performance of a high voltage cable termination.
The design tests of the IEEE procedure that are particularly useful in determining the effectiveness of a termination which includes a stress control arrangement include the "Partial Discharge (Corona) Extinction Voltage Test" and the "Lightning Impulse Voltage Withstand Test". In the discharge extinction voltage test, electrical discharge in the termination is measured at specific applied voltages and has to be below specific values. Also the voltage at which the discharge extinguishes is measured and has to be above specific values. In the impulse voltage withstand test, impulses of specific value and waveform are applied to the cable and should be withstood without flashover. The voltage at which flashover occurs should be above specific values. Both the discharge and impulse performance of the termination should meet the requirements set forth in the IEEE Standard Test procedures STD 48-1975.
The use of semiconductive stress control material in high voltage cable terminations does not always produce termination that meets the impulse performance requirements of the IEEE test procedures. In order to meet this requirement the stress control arrangement may be augmented by the use of sheds. While sheds are typically employed with outdoor terminations for other purposes, they are not generally employed when the cable termination is installed indoors. Since the use of sheds adds to the cost of the termination and requires additional space around the cable, it is desirable to be able to dispense with the use of the sheds yet still meet the desired impulse performance.
The present invention, provides a novel arrangement and method that retains the the electrical stress control capabilities of the semiconductive stress control material while significantly improving both its discharge and impulse performance without the use of sheds. While the present invention is primarily described in connection with a termination of a cable, it is suitable for employment with high voltage cable joints and other high voltage equipment including electrical bushings and feed throughs.